Valence bond theoretical study for chemical reactivity

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Elastic and charge transfer processes in H+ + CO collisions

Proton and hydrogen atom time-of-flight spectra in collision energy range of E(trans) = 9.5-30 eV show that the endoergic charge transfer process in the H+ + CO system is almost an order of magnitude less probable than the elastic scattering [G. Niedner-Schatteburg and J. P. Toennies, Adv. Chem. Phys. LXXXII, 553 (1992)]. Ab initio computations at the multireference configuration interaction level have been performed to obtain the ground- and several low-lying excited electronic state potential energy curves in three different molecular orientations namely, H+ approaching the O-end and the C-end (collinear), and H+ approaching the CO molecule in perpendicular configuration with fixed CO internuclear distance. Nonadiabatic coupling terms between the ground electronic state (H+ + CO) and the three low-lying excited electronic states (H + CO+) have been computed and the corresponding diabatic potentials have been obtained. A time-dependent wavepacket dynamics study is modeled first involving only the ground and the first excited states and then involving the ground and the three lowest excited states at the collision energy of 9.5 eV. The overall charge transfer probability have been found to be approximately 20%-30% which is in qualitative agreement with the experimental findings.     

Vibrational mode and collision energy effects on reaction of H2CO+ with C2H2: charge state competition and the role of Franck-Condon factors in endoergic charge transfer

The effects of collision energy (E(col)) and six different H(2)CO(+) vibrational states on the title reaction have been studied over the center-of-mass E(col) range from 0.1 to 2.6 eV, including measurements of product ion recoil velocity distributions. Ab initio and Rice-Ramsperger-Kassel-Marcus calculations were used to examine the properties of complexes and transition states that might be important in mediating the reaction. Reaction is largely direct, despite the presence of multiple deep wells on the potential surface. Five product channels are observed, with a total reaction cross section at the collision limit. The competition among the major H(2) (+) transfer, hydrogen transfer, and proton transfer channels is strongly affected by E(col) and H(2)CO(+) vibrational excitation, providing insight into the factors that control competition and charge state "unmixing" during product separation. One of the more interesting results is that endoergic charge transfer appears to be controlled by Franck-Condon factors, implying that it occurs at large inter-reactant separations, contrary to the expectation that endoergic reactions should require intimate collisions to drive the necessary energy conversion.     

Identification of destabilized metal hydrides for hydrogen storage using first principles calculations

Hydrides of period 2 and 3 elements are promising candidates for hydrogen storage but typically have heats of reaction that are too high to be of use for fuel cell vehicles. Recent experimental work has focused on destabilizing metal hydrides through alloying with other elements. A very large number of possible destabilized metal hydride reaction schemes exist. The thermodynamic data required to assess the enthalpies of these reactions, however, are not available in many cases. We have used first principles density functional theory calculations to predict the reaction enthalpies for more than 100 destabilization reactions that have not previously been reported. Many of these reactions are predicted not be useful for reversible hydrogen storage, having calculated reaction enthalpies that are either too high or too low. More importantly, our calculations identify five promising reaction schemes that merit experimental study: 3LiNH(2) + 2LiH + Si --> Li(5)N(3)Si + 4H(2), 4LiBH(4) + MgH(2) --> 4LiH + MgB(4) + 7H(2), 7LiBH(4) + MgH(2) --> 7LiH + MgB(7) + 11.5H(2), CaH(2) + 6LiBH(4) --> CaB(6) + 6LiH + 10H(2), and LiNH(2) + MgH(2) --> LiMgN + 2H(2).     

Non-Born-Oppenheimer variational calculation of the ground-state vibrational spectrum of LiH+

Very accurate, rigorous, variational, non-Born-Oppenheimer (non-BO) calculations have been performed for the fully symmetric, bound states of the LiH(+) ion. These states correspond to the ground and excited vibrational states of LiH(+) in the ground (2)Sigma(+) electronic state. The non-BO wave functions of the states have been expanded in terms of spherical N-particle explicitly correlated Gaussian functions multiplied by even powers of the internuclear distance and 5600 Gaussians were used for each state. The calculations that, to our knowledge, are the most accurate ever performed for a diatomic system with three electrons have yielded six bound states. Average interparticle distances and nucleus-nucleus correlation function plots are presented.     

Evidence for long-lived excited states of [CnH2]2+ carbodications

The energetics, structures, stabilities and reactivities of[C_nH₂]²⁺ ions have been investigated using computational methods and experimental mass spectrometric techniques. Spontaneous decompositions of [C_nH₂]²⁺ into [C_nH]⁺ + H⁺ products, observed for ions with odd-n values, have been explained by invoking the formation of excited triplet states. Even-n [C_nH]⁺ ions possess triplet ground states with low-lying excited states, whereas odd-n ions have triplet states with energies several eV above ground singlet states. Radiationless transitions of vibrationally excited long-lived triplet state ions into singlet state continua are suggested as possible mechanisms for spontaneous deprotonation processes of odd-n [C_nH₂]²⁺ ions. Evidence for these long-lived excited states has been obtained in bimolecular single electron transfer reactions.     

Accurate Ab Initio Calculations for LiH and its Ions, LiH+ and LiH−

Ab initio calculations were performed for LiH using a pseudopotential approach with CPP corrections and huge basis sets on both atoms. A wide range of ^1,3Σ⁺ electronic adiabatic states have been investigated, from the ground state up to those dissociating into Li(5p)+H. Permanent and transition electric dipole moments are also considered for the first few excited states. Comparison with experiments and recent all-electron calculations, reveals an excellent global accuracy, only the bottom of the ground state being better described by all-electron approaches. Using almost identical basis sets, coupled cluster all-electron calculations are performed for the ground states of LiH⁺, LiH⁻ and LiH. High care has been given to the correct relative position of the asymptotes, allowing for this rather complete set of accurate ab initio data to be useful for further molecular physics studies.     

LiH分子X 1Σ+ 、A 1Σ+和B 1Π态的势能函数

The energies, equilibrium geometries and harmonic frequencies of the three electronic states (the ground state X 1Σ+, the first excitation state A 1Σ+ and the second excitation degenerate state B 1Π) of LiH molecule have been calculated by using the GSUM (Group Sum of Operators) method of SAC/ SAC-CI with the basis sets D95(d), 6-311G**, and cc-PVTZ. Comparing with the above-mentioned three basis sets, the conclusion is gained that the basis set D95(d) is the most suitable for the energy calculation of LiH molecule. The whole potential curves for these three electronic states are further scanned, using SAC/D95(d) method for the ground state and SAC-CI/D95(d) methods for the excited states. Murrell-Sorbie function were fitted using a least square and then the spectroscopy constants are calculated, which are in good agreement with the experimental data.     

Binding energy curves from nonempirical density functionals. I. Covalent bonds in closed-shell and radical molecules

Binding or potential energy curves have been calculated for the ground-state diatomics H(2)(+), He(2)(+), LiH(+), H(2), N(2), and C(2), for the transition state H(3), and for the triplet first excited state of H(2) using the nonempirical density functionals from the first three rungs of a ladder of approximations: the local spin density (LSD) approximation, the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA), and the Tao-Perdew-Staroverov-Scuseria (TPSS) meta GGA. Good binding energy curves in agreement with coupled cluster or configuration interaction calculations are found from the PBE GGA and especially from the TPSS meta GGA. Expected exceptions are the symmetric radicals H(2)(+) and He(2)(+), where the functionals suffer from self-interaction error, and the exotically bonded C(2). Although the energy barrier for the reaction H(2) + H --> H + H(2) is better in PBE than in TPSS, the transition state H(3) is a more properly positioned and curved saddle point of the energy surface in TPSS. The triplet first excited state of H(2) obeys the Aufbau principle and thus is one of the exceptional excited states that are computable in principle from the ground-state functional. The PBE GGA and TPSS meta GGA are useful not only for chemical applications but also for the construction of higher-rung nonempirical functionals that can further improve the binding energy curves.     

Structures and binding energies of the (dibenzoylmethanato)boron difluoride complexes with aromatic hydrocarbons in the ground and excited states. Density functional theory calculations

Structures of the (dibenzoylmethanato)boron difluoride molecule (DBMBF2) and its complexes with a series of aromatic hydrocarbons (benzene; toluene; o-, m-, and p-xylenes, naphthalene; anthracene; and pyrene) in the ground and the first singlet excited states have been calculated. The calculations have been performed by the density functional theory (DFT) and time-dependent density functional theory (TDDFT) for the ground and excited states, respectively, with the empirical dispersion correction. It has been shown that the complexes in the ground and excited states have similar stacking structures and are characterized by short contacts between the F atom of DBMBF2 and H atoms of the hydrocarbon molecule, which decrease on transition from the ground to the excited state. The calculated binding energies in the complexes in the excited state are two to three times higher than those in the ground state. The charge transfer in the ground state of the complexes is insignificant and directed from DBMBF2 to the ligand, while in the excited state it is 0.6–0.8 e and directed from the ligand to DBMBF2.     

Stability and structure of Li(n)H molecules (n=3-6): experimental and density functional study

The molecules Li(3)H and Li(4)H have been identified in mass-spectrometric measurements over solutions of hydrogen in liquid Li, and the gaseous equilibria of the reactions: Li(3)H+Li=Li(2)H+Li(2), Li(3)H+Li(2)=Li(2)H+Li(3), Li(3)H+Li=LiH+Li(3), Li(3)H+LiH=2Li(2)H, and Li(4)H+Li(2)=Li(3)H+Li(3) have been measured. Density functional calculations of Li(n)H molecules (n=3-6) provide structures, vibrational frequencies, ionization energies, and free energy functions of these molecules, and these are used to estimate the enthalpies of these reactions and the atomization energies of Li(3)H (119.4 kcal/mol) and Li(4)H (151.8 kcal/mol).     

State-selective electron capture processes in collisions involving boron ions

A full theoretical treatment of electron capture processes using ab initio configuration interaction methods within, according to the collision energy range concerned, a semiclassical or a quantal collisional formalism including translation effects has been developed recently. An application for collisions involving boron, an important impurity in fusion reactors, is presented on examples of the ground state: B³⁺(1s²) + He, B⁴⁺(1s) + H, and the metastable ion B³⁺(1s2s) + H reactions. © 1996 John Wiley & Sons, Inc.     

H_3~+体系基态和电子激发态的价键方法研究

本文应用价键理论的键表酉群方法,讨论H_3~+体系的基态及其几个低激发态,研究了该体系处于激发态时价键结构的变化,得到H_3~+体系的基态为等边三角形,第一激发态为直线型,第二、三激发态为等腰三角形,且基本简并.     

The polarization dependent differential cross sections of the reactions:H+LiH~+(v=0,j=0)→H_2+Li~+ and H~++ LiH(v=0,j=0)→H_2~++Li

<正>Quasi-classical trajectory(QCT) calculations have been carried out to study the generalized polarization dependent differentialcross sections(PDDCSs) for the reactions H + LiH~+(v = 0,j = 0)→H_2 + Li~+ and H~+ + LiH(v = 0,j = 0)→H_2~+ + Li occurring onthe two lowest-lying electronic states of the LiH_2~+ system,using the ab initio potential energy surfaces(PESs) of Martinazzo et al.[3].Four PDDCSs,i.e.,(2π/σ)(dσ_(00)/dω_t),(2π/σ)(dσ_(20)/dω_t),(2π/σ)(dσ_(22+)/dω_t),(2π/σ)(dσ_(21-)/dω_t) have been discussed in detail.     

Direct dynamics classical trajectory simulations of the O+ + CH4 reaction at hyperthermal energies

A Born-Oppenheimer direct dynamics simulation of the O(+) + CH(4) reaction dynamics at hyperthermal energies has been carried out with the PM3 (ground quartet state) Hamiltonian. Calculations were performed at various collision energies ranging from 0.5 to 10 eV with emphasis on high energy collisions where this reaction is relevant to materials erosion studies in low Earth orbit and geosynchronous Earth orbit. Charge transfer to give CH(4)(+) is the dominant channel arising from O(+) + CH(4) collisions in this energy range, but most of the emphasis in our study is on collisions that lead to reaction. All energetically accessible reaction channels were found, including products containing carbon-oxygen bonds, which is in agreement with the results of recent experiments. After correcting for compensating errors in competing reaction channels, our excitation functions show quantitative agreement with experiment (for which absolute magnitudes of cross sections are available) at high collision energies (several eV). More detailed properties, such as translational and angular distributions, show qualitative agreement. The opacity function reveals a high selectivity for producing OH(+) at high impact parameters, CH(3)(+)/CH(2)(+)/H(2)O(+) at intermediate impact parameters, and H(2)CO(+)/HCO(+)/CO(+) at small impact parameters. Angular distributions for CH(3)(+)/CH(2)(+)/OH(+) are forward scattered at high collision energies which implies the importance of direct reaction mechanisms, while reaction complexes play an important role at lower energies, especially for the H(2)O(+) product. Finally, we find that the nominally spin-forbidden product CH(3)(+) + OH can be produced by a spin-allowed pathway that involves the formation of the triplet excited product CH(3)(+)(?(3)E). This explains why CH(3)(+) can have a high cross section, even at very low collision energies. The results of this work suggest that the PM3 method may be applied directly to the study of O(+) reactions with small alkane molecules and polymer surfaces.     

The role of the excited electronic states in the C+ + H2O reaction

The electronic excited states of the [COH2]+ system have been studied in order to establish their role in the dynamics of the C+ + H2O-->[COH]+ +H reaction, which is a prototypical ion-molecule reaction. The most relevant minima and saddle points of the lowest excited state have been determined and energy profiles for the lowest excited doublet and quartet electronic states have been computed along the fragmentation and isomerization coordinates. Also, nonadiabatic coupling strengths between the ground and the first excited state have been computed where they can be large. Our analysis suggests that the first excited state could play an important role in the generation of the formyl isomer, which has been detected in crossed beam experiments [D. M. Sonnenfroh et al., J. Chem. Phys. 83, 3985 (1985)], but could not be explained in quasiclassical trajectory computations [Y. Ishikawa et al., Chem. Phys. Lett. 370, 490 (2003); J. R. Flores, J. Chem. Phys. 125, 164309 (2006)].     

Electron ionization of methane: the dissociation of the methane monocation and dication

Time-of-flight mass spectrometry and two-dimensional coincidence techniques have been used to determine, for the first time, the relative precursor-specific partial ionization cross sections following electron-methane collisions. Precursor-specific partial ionization cross sections quantify the contribution of single, double, and higher levels of ionization to the partial ionization cross section for forming a specific ion (e.g. CH(+)) following electron ionization of methane. Cross sections are presented for the formation of H(+), H(2)(+), C(+), CH(+), CH(2)(+), and CH(3)(+), relative to CH(4)(+), at ionizing electron energies from 30 to 200 eV. We can also reduce our dataset to derive the relative partial ionization cross sections for the electron ionization of methane, for comparison with earlier measurements. These relative partial ionization cross sections are in good agreement with recent determinations. However, we find that there is significant disagreement between our partial ionization cross sections and those derived from earlier studies. Inspection of the values of our precursor-specific partial ionization cross sections shows that this disagreement is due to the inefficient collection of energetic fragment ions in the earlier work. Our coincidence experiments also show that the lower energy electronic states of CH(4)(2+) populated by electron double ionization of CH(4) at 55 eV are the same (ground (3)T(1), first excited (1)E(1)) as those populated by 40.8 eV photoionization. The (3)T(1) state dissociating to form CH(3)(+) + H(+) and CH(2)(+) + H(2)(+) and the (1)E(1) to form CH(2)(+) + H(+) and CH(+) + H(+). At this electron energy, we also observe population of the first excited triplet state of CH(4)(2+) ((3)T(2)) which dissociates to both CH(2)(+) + H(+) + H and CH(+) + H(+) + H(2).     

Crossed beam studies of the reactions of atomic oxygen in the ground 3P and first electronically excited 1D states with hydrogen sulfide

The reactions of both ground, (3)P, and electronically excited, (1)D, oxygen atoms with hydrogen sulfide, H(2)S, have been investigated by means of the crossed molecular beams method with mass spectrometric detection at different collision energies. Amongst the possible reaction channels those leading to HSO+H for the O((3)P) reaction and to HSO/HOS+H and SO+H(2) for the O((1)D) reaction have been identified and investigated. The dynamics of the channels leading to HSO/HOS+H are elucidated for the reactions of both states and the trend with increasing the collision energy analyzed. Noteworthily, the formation of SO+H(2) products appears to be an open channel for the O((1)D) reaction, at least for the highest collision energy investigated (11.8 kcal/mol). Finally, the recent experimental and theoretical estimates of the enthalpy of formation of the HSO radical have been critically analyzed to evaluate their conformity with the present experimental data.     

Product branching between reactive and nonreactive pathways in the collisional quenching of OH A 2Sigma+ radicals by H2

The collisional quenching of OH radicals in their excited A 2Sigma+ electronic state by molecular hydrogen is examined to determine the partitioning between reactive and nonreactive pathways. This is achieved using a pump-probe laser technique to compare the population prepared in the excited OH A 2Sigma+ state with that produced in the OH X 2Pi ground state from nonreactive quenching. Only a small fraction of the products, less than 15%, arise from nonreactive quenching; reactive quenching is the dominant product channel. The branching between the product channels provides a new dynamical signature of the conical intersection region(s) that couple the excited state potential for OH A 2Sigma++H2 with OH X 2Pi+H2 and H2O+H products.     

Hydrogen desorption mechanism in a Li-N-H system by means of the isotopic exchange technique

The hydrogen desorption mechanism in the reaction from LiH + LiNH2 to Li2NH + H2 was examined by thermal desorption mass spectrometry, thermogravimetric analysis, and Fourier transform IR analyses for the products replaced by LiD or LiND2 for LiH or LiNH2, respectively. The results obtained indicate that the hydrogen desorption reaction proceeds through the following two-step elementary reactions mediated by ammonia: 2LiNH2 --> Li2NH + NH3 and LiH + NH3 --> LiNH2 + H2, where hydrogen molecules are randomly formed from four equivalent hydrogen atoms in a hypothetical LiNH4 produced by the reaction between LiH and NH3 according to the laws of probability.